Conventionally, for example, the amount of air sucked into an engine is controlled by adjusting the angular aperture of a butterfly valve mounted on a throttle body, which is executed by the switching drive of a DC motor. In the case, the butterfly valve on the throttle body is driven by the output torque of the DC motor, which is amplified by eleven times using a double reduction mechanism with two sets of gears, and the angular aperture of the valve is sensed using a potential angle detector composed of a thin film resistor and a set of metal brushes. However, in the case that a DC motor is used as a driver, a backlash resulted from the gears is inevitable, so that the angular aperture of the valve is difficult to be accurately controlled. Moreover, the detrimental effect on the durability, the lifetime, and the accuracy caused by the slide of the metal brushes on the surface of the thin film resistor is also inevitable in the potential angle detector.
On the other hand, to use rotary actuators, which require no gears causing a backlash, for rotation control is considered. For example, in Japanese Unexamined Patent Application No. Hei 9-163708, a rotary actuator is proposed in which a stator coil is wound around a stator core having two magnetic poles as a stator, a cylindrical rotor is provided around the stator, and two permanent magnets are secured on the internal surface of the rotor so as to face the above stator core. In this conventional technique, the thickness of both end portions of each magnet is set to be not more than 90 percent of that of the central portion. By controlling the thickness of the permanent magnets in this manner, the rotor can be reliably rotated only to two target positions without generating the opposite torque during supplying no electric current.
In the rotary actuator by the above conventional technology, the rotor stops at an initial position when not supplying an electric current, and rotates to a predetermined angular position when supplying an electric current, and returns to the initial position by the torque of the permanent magnets when the supply of an electric current is stopped. Due to this, the rotor cannot be stopped and held at any position apart from above two angular positions. Therefore, it is impossible to control the angular aperture of the valve for adjusting the above amount of the air sucked into the engine using the rotary actuator by the above conventional technology.